Posted
by
samzenpus
on Tuesday September 22, 2015 @05:44PM
from the looking-for-the-answers dept.

schwit1 writes: The hunt for gravitational waves began again for the Laser Interferometer Gravitational-Wave Observatory (LIGO)-the largest instrument of its kind. The restart follows a five-year-long, US $200-million project to overhaul the experiment's detectors. Many physicists believe the revamped experiment, dubbed Advanced LIGO, will be the first to find direct evidence of gravitational waves: ripples in the fabric of space-time that can be created by, among other things, a pair of neutron stars or black holes orbiting each other.
Gravitational waves were first theorized in 1916 by Albert Einstein as a consequence of his general theory of relativity, which celebrates its centennial this year.

I'm not trolling here. These are honest questions: I assume, since we're spending more money on a more advanced instrument, that we didn't find anything the first time around? Was that because the instrument likely wasn't sensitive enough or because they likely don't exist? If we didn't find them first time around, does that call into question some aspect of GR? I know GR is a theory that has been well proven, but if we don't find them this time around does that have significant implications?

If I remember correctly, the noise floor of the previous instrument was approximately the level of the signal they were looking for.
A better detector may help.

Indeed. It's hard to overstate the sensitivity of these instruments, or the vulnerability of these instruments to noise. To take one example, here's an ArXiv preprint [arxiv.org] that calculates that the original LIGO detectors would need to be physically shielded from tumbleweeds, since the the impact of a wind-borne tumbleweed on the building exterior (100 feet from the detector) could produce a vibrational or gravitational transient sufficient to appear to be a spurious gravitational wave signal.

That sounds somewhat bogus. Didn't the "scientists" who built the first experiment know what the noise issues were and that the first equipment couldn't find anything? Or to put it another way, would they have known if they were building it with their own money rather than grant money?

And how would this new experiment establish that it was really seeing gravity waves from distant sources rather than much closer sources, such as ocean waves or birds flying nearby or any heavy equipment moving around on th

Just speculating here, but that is likely why there are two detector stations; one on either side of the country. An event showing up at both detectors nearly simultaneously would likely be external waves. Events showing up at only one detector could be written off as local noise.

Just speculating here, but that is likely why there are two detector stations; one on either side of the country. An event showing up at both detectors nearly simultaneously would likely be external waves. Events showing up at only one detector could be written off as local noise.

Exactly, It is like a differential amplifier, both detectors playing the role of the positive and negative inputs, noise at other input is cancelled out by it's absence at the other detector, thereby amplifying the common signal.

Also it is worthy of noting that the needed sensitivity was not exactly known when the first LIGO was built.

In Engineering it is a general rule that your detection or control apparatus have at least an order of magnitude of sensitivity or resolution to see or control the thing you a

Another reason: assuming there is good clock synchronization between the data feeds of the two sites, you should be able to use their separation to determine from where in the galaxy the wave came from. (With two sites, you can only narrow down the location so far. Three sites should provide a good fix; N sites would be that much better.)

Two detectors would give you a plane projected on the sky (like the celestial equator, or the plane of the galaxy).

Three detectors would effectively give you two such planes, which in most cases would intersect at two antipoal points on the plane of the sky. (You can constrain that by the orientation and placing of the detectors, but Muphry's Law dictates that the first detection would be on two planes so close to parallel that Slashdot would explode with indignation at the incompetence of the design)

Didn't the "scientists" who built the first experiment know what the noise issues were and that the first equipment couldn't find anything?

There is a range of possible intensities at a given frequency for predicted gravitational waves. The original LIGO project overlapped with potential ranges, so there was a potential possibility of seeing something. It was not built with the certain expectation it would find nothing.

This chart [wikipedia.org] does a really good job of summarizing different predicted sources of gravitational waves, and the sensitivity of current and proposed detectors.

This is one of the really useful experiments that could be more easily done in space. (As opposed to, for example, most of what the space station is used for.)

A laser interferometer in interplanetary space could have an enormous path length quite easily, and would not sense all the vibrations on Earth. It could also be in 3-dimensions, consisting of a satellite hub and 3 corner-cube mirrors at long distances from the hub.

That is the whole point of the soon to be launched LISA pathfinder project. It will have an arm-length on the order of a million kilometers. The long arm length and lack of seismic noise make it very sensitive to frequencies several orders of magnitude below that of LIGO. However, the long arm length also makes it insensitive to higher frequencies (when the wavelength is smaller than the arm length). So it works out such that LISA and LIGO frequency ranges don't overlap, and the complement each other qu

It will be the point of the LISA project, with 3 spacecraft and a million-km arm length. LISA Pathfinder is a test mission for the detector technology, this mission packs the experiments in a single spacecraft.

A laser interferometer in interplanetary space could have an enormous path length quite easily, and would not sense all the vibrations on Earth. It could also be in 3-dimensions, consisting of a satellite hub and 3 corner-cube mirrors at long distances from the hub

Mostly correct. One of the main hurdles, however, is controlling the positions of the spacecraft relative to each other to extremely tight tolerances. In deep space this isn't too difficult. In Earth or Lunar orbit, it's quite difficult. An

If I remember correctly, the noise floor of the previous instrument was approximately the level of the signal they were looking for.A better detector may help.

As someone who used to work at LIGO Hanford (quite some time ago), I can confirm this. It was always planned to be two phases. The first stage was simpler, and was used to get through any teething issues. It actually used simpler mirror controls and detection systems than many other gravitational wave detectors around the world, and made up the sensitivity by being much longer (4km beam length compared to some that were just 300m). That let it get up to speed quickly with at least some chance of still being able to detect something. However, the only way they expected to get a good signal was either by being lucky, or if the estimates of the actual signal level were off and there were much stronger signals out there. But, as the site director I worked for liked to point out, every time we have opened up a new way to view the universe, we saw something unexpected. Optical telescopes, UV, IR, radio, etc. all saw something new. If there is something we don't expect out there, it might send a strong signal.

Since they didn't get lucky and get a clear signal, they moved on to the second phase and replaced the simple control systems with the more complex (and likely more fiddly) systems. Since a large part of the cost of the project was "baking" the 4km steel tubes in order to get a good enough vacuum, the upgrade of the mirrors and control systems was comparatively cheap. The advanced mirror controls are expected to match or exceed the detectors elsewhere, and combined with the much longer beam tubes it should show sensitivity far beyond anything else out there.

As an interesting side note, some parts of the Advanced LIGO upgrades have been installed at the Louisiana site for quite some time.. Specifically, the seismic isolation systems were upgraded soon after it came online. When surveys were initially done for the site, the seismic noise level was just fine, but soon after the interferometer came online the forest around them matured to the point where logging operations commenced. Even miles away, trees falling to the ground were enough to shake the instrument out of lock. When looking at plots of when the LA site was collecting data, you could clearly see dawn and dusk, because as soon as there was enough light to chop down a tree, the instrument became useless. This forced them to move up plans for the active isolation system originally scheduled for Advanced LIGO, and they installed it to replace the passive isolation system. It actually worked rather well, and gave them some experience with the system, which hopefully helped with installing it at the Hanford site.

If gravity waves don't actually exist it would be the first prediction of General Relativity to fail, which would be a huge discovery that could help in the search for quantum gravity. Scientists have indirect [cardiff.ac.uk] evidence of gravity waves:

In this current "pre-detection" era it can be difficult to convince those who are not overly familiar with the theory of general relativity that gravitational waves really do exist. Fortunately, the Hulse-Taylor Pulsar (PSR 1913+16) provides firm evidence of a binary system

If we can't measure it, it does not mean that they do not exist. They could be weaker to some extent, yes, but it could be also that the "sources" are so evenly distributed around us that the superposition of their waves at our location almost cancel itself.

This is referred to as the stochastic background: The cumulative sum of all the waves and sources we can't individually make out, and the remnant metric fluctuations from the birth of the universe that will wobble around for eternity.

The sensitivity floor at ALIGO's best frequencies is below nearly all realistic predictions of the stochastic background.

Bottom line, if ALIGO doesn't see *something*, something is horribly wrong either at ALIGO or with our understanding of Relativity.

The expected main sources of gravity waves are things like merging binary "star" systems where the stars are actually blackholes or neutron stars, and supernova explosions. However, these are relatively rare events.So, for a gravity wave detector to see something, such an event must take place within the volume of spacewhere the detector has the sensitivity to detect something. That means for the original LIGO to detect somethingwe would have had to have been very lucky to have seen something.With the upgraded version, the volume of space where LIGO will be sensitive is greatly expanded.We have educated guesses for e.g. the occurrence rate of merging black holes. That can be usedto estimate how likely it is that a gravity wave detection would be made within a certain period of timeThe current estimates give advanced LIGO a good chance of detecting something. (I'm too lazyto check the actual numbers!)So, if nothing is seen within a few years at the final sensitivity limit then people will have to reexaminetheir estimates of event rates and/or general relativity.

Tricky as it is to create a gravitational detector, a gravitational radiator the emits significant power is a lot tougher. I seem to remember that thermonuclear bombs and asymmetric explosions generate a trivial amount of gravitational wave energy.

LIGO is looking for supernova scale sources - probably not a good idea to build one in our solar system.

Yes, that is one of the important characteristic of it being a radiated wave. The wave traveling away carries energy, whether or not there is something to interact with it. If gravity propagated instantly such that there were no waves, then the only way energy would be loss would be through interactions with other stuff. Evidence supports the former, considering we see pulsars with decaying orbits closely matching GR predictions and nothing in their vicinity.

Well, not anything that moves, but things that move with a quadrupole moment. In other words, spherically and cylindrical symmetric movements do not radiate. A mass moving linearly by itself, or a sphere spinning will not radiate. However, two masses in orbit around each other will.

Given that gravitational-waves have a vector, and with so many body's radiating in the plan's direction, would not this make for a stronger gravity in the rotation plain of the Galaxy, and account for why start's do not fly off ?
Lachlan

GR requires them to exist. I don't know if there are other gravity theories that are consistent with all other observations that do not.

We do see spin-down of binary neutron stars that is consistent with gravitational wave radiation, so its pretty darn clear that they do exist - we just don't have direct detection.

In the future LISA ( http://lisa.nasa.gov/ [nasa.gov] ) and other more advanced instruments may be able to to gravity wave astronomy. Ultimately we could imagine detecting gravitational radiation from t

I have a slight problem understanding how gravitational waves would necessarily be detectable. We know, by href="https://en.wikipedia.org/wiki/Landauer%27s_principle">Landauer's principle, that any erasure of a bit of information must be accompanied by a corresponding increase in entropy. Arguably, a neutron star / pulsar slowly inspiraling is a source of increasing entropy, i.e. information is being erased by its doing so. Until now, we have always seen this increase of entropy as a release of energy ("

GR does not necessarily "oblige" gravitational waves to be carriers of information

It does, through the mass-energy-momentum conservation ${\nabla^\mu}T_{\mu\nu} = 0$ (in geometrized units where G=c=1=\pi). As dynamical spacetime (T_munu) moves around objects with mass-energy-momentum (e.g. the quantum field content in semiclassical gravity), the latter responds in a completely unambiguous way, just as a change in the non-gravitational field content causes spacetime to respond in a completely unambiguous wa

I'm not trying to knock you or anything, but your post did remind me of a discussion I had with the LIGO Hanford director (when I worked there a while back). There are many people who don't understand the details of gravitational wave theory (me included), and most of them are either indifferent or curious. They either don't care too much, or ask questions from experts in order to expand their knowledge (like you seem to be doing). Then there are the other types, who either want to prove the experts wron

I don't really see how this method can even detect the gravity waves. As the gravity waves come along, they change the length of the beams. But the change in gravity will also change time. So the light beam traveling down the beam will appear to have taken longer to travel the shorter distance. I bet they cancel each other out and you have no difference in the time taken to travel down the beams even when there is a gravity wave traveling by.